pro training split?

when i read professional bodybuilder routines like arnolds (day1 :chest,back,legs ... day2 shoulders,bi's,tri's) they dont really make any sence to me ! first on day1 why would you train those 3 huge muscle groups in day1 together because if you're gonna train each body part 3 times a week, day1 would be a killer plus on day 2 your bi's and tri's will only have like 12 hours to recover! and dont get me wrong i know that these prro routines are called pro and advanced routines for a reason im just curious! why wouldnt they go with something like this ... day1 : chest,tri's,legs
day2 : back,shoulders,bi's

ive tried this routine with 3 exercises at 3-4 sets of each exercise for big muscle groups and 2 exercises at 3-4 sets of each exercise and i thought i would be overtraining for sure but i just wanted to see for myself and i get amazing gains from this routine! maybe im one of a kind because im a hard gainer and they say hard gainers need alot of recovery time but for me it seems like within 48 hours im good to work that bodypart out again at a high intensity level !

that's awesome bro. I've tried the arnold routine and I definitely dont have 3 hours to spare in the gym so i stopped it. web

well what i did was at first was only 3 exercises for a big muscle (chest) and 2 exercises for a small muscle (bi's) and only like 3 sets !